Carbohydrate-rich materials such as naturally occurring lignocellulosic biomass (wood, agricultural residues, etc.) or manmade products such as size-reduced softwood, paper, cardboard, and so on are an important source of non-food simple sugars, oligomers, and polymers for a variety of industrial applications. These materials consist primarily of lignocellulosic fibers which are typically comprised of cellulose, hemicellulose, and lignin. Cellulose is a polymer of glucose, hemicellulose is a polymer of a variety of hexose and pentose sugars (primarily mannose in softwood and xylose in other biomass), and lignin is an aromatic polymer. The main hexose sugars in biomass are glucose, mannose and galactose, and the main pentose sugars in biomass are xylose and arabinose. The relative amount of cellulose, hemicellulose and lignin of various materials is highly dependent on the specific type of biomass.
Regardless of its type, biomass is composed of fibers “glued” into bundles. The glue is primarily lignin, and is concentrated in the compound middle lamella (the outer layer of fibers). Much of the cost of extracting sugars from biomass using existing methods is in separating fibers and removing (or relocating) the lignin on the outside of the fibers so that the cellulose and hemicellulose in the fibers are more easily accessible from the outside of the fibers. For instance, making paper pulp (separated fibers) from wood chips is very energy-intensive, requiring rather extreme operating conditions and in the case of Kraft or sulfite pulp, also harsh chemicals. Similarly, the energy required to separate fibers using steam explosion, ammonia fiber expansion, or other pretreatment techniques make it cost prohibitive to process biomass into separate fibers amenable for sugar extraction.
Fibers are hollow, usually filled with air, typically with a roughly round profile. The hollow part of a fiber is called the lumen, and is typically about 36% of the volume of a fiber. The density of the cell wall is roughly 1.5 g/cm3, and the overall density of most fibers (when the air-filled center is taken into account) is roughly 0.96 g/cm3. For this reason, most dry or partially moist biomass particles generally float in water. The air pockets in biomass are not easily displaced with water, which is why wood or straw bales will float for weeks or months before sinking.
Fibers generally are between 1 and 4 mm long and about 20 to 40 microns in diameter, depending on the type of plant that produced them. The walls of these fibers contain a large number of holes, called pits or pores, ranging in size from 30 nm to 1000 nm. The cell wall is about 20% of the diameter of a fiber, resulting in the lumen having about 36% of the volume of a typical fiber. The characteristics of biomass are described in more detail in Gibson, “The hierarchical structure and mechanics of plant materials,” Journal of The Royal Society Interface 9 76 (2012): 2749-2766, which is hereby incorporated by reference herein.
The inner surface of the cell wall of fibers, starting at the lumen, has the lowest concentration of lignin in fibers. The distribution of cellulose, hemicellulose, and lignin in plant cell walls is described in detail by Gierlinger et al., “Raman Imaging of Lignocellulosic Feedstock,” in Cellulose—Biomass Conversion, edited by van de Ven and Kadla (2013): 159, and in Gierlinger, “Revealing changes in molecular composition of plant cell walls on the micron-level by Raman mapping and vertex component analysis (VCA),” Frontiers in plant science 5 (2014), both of which are hereby incorporated by reference herein.
The degree of polymerization (DP) is defined as the number of monomeric units in a macromolecule. In the case of cellulose and hemicellulose, the monomeric units are simple sugars. In biomass, reducing the degree of polymerization of cellulose and hemicellulose can be done with reagents that cause hydrolysis and/or oxidation reactions. The cellulose and hemicellulose contained in fibers can be hydrolyzed to hexose monomeric sugars such as glucose and mannose and to pentose monomeric sugars such as xylose and arabinose. Cellulase enzymes, hemicellulase enzymes, dilute acid solutions, strong acids, and strong bases catalyze these hydrolysis reactions. Hydrolysis also can be carried out at high temperatures in the absence of a catalyst. The degree of polymerization of cellulose and hemicellulose can also be reduced by oxidation with a Fenton or Fenton-like reagent, composed of a transition metal catalyst in solution with hydrogen peroxide.
Sugars have significant economic value and can be fermented to liquid fuels such as ethanol, butanol, or other specialty chemicals and can also be used for animal nutrition. Polymeric sugars, such as nanocellulose crystals, have increasingly considerable industrial interest and economic value because of possible application in the development of novel high performance renewable materials. Lignin and sugar monomers are not significantly degraded at 90° C. at a pH of 1.0 and above, but oligomers and polymers of cellulose and hemicellulose are randomly hydrolyzed. Amorphous regions of cellulose are quickly hydrolyzed at a pH less than 2 and a temperature of about 90° C., as is hemicellulose (which is also amorphous). Crystalline regions of cellulose fibrils are not significantly degraded at a pH above 1.0 and 90° C. because of the hydrogen bonds between cellulose chains in crystalline cellulose.
When hydrolyzing biomass to produce sugars, one of the fundamental limitations involved is mass transfer of sugars out of the bulk fibers, due to limitations of simple diffusion. A solution is desired. What is especially needed in the biorefining industry is a method to reduce the cost of extracting sugars from biomass by eliminating the costly step of separating the fibers and relocating the lignin on the outside of the fibers. It is also highly desirable to reduce the high capital and operating costs associated with biomass size reduction and mechanical mixing during hydrolysis.